Re: [Xen-devel] Future support of 5-level paging in Xen:wq

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Dec 13, 2016, at 12:16 PM, Tian, Kevin <kevin.tian@xxxxxxxxx> wrote:
> 
>> From: Li, Liang Z
>> Sent: Tuesday, December 13, 2016 11:58 AM
>> 
>> Hi All,
>> 
>> We are now working on enabling 5 level paging & 5 level EPT for XEN. We need 
>> the
>> community's opinions about the following aspects:
>> 
>> 1.  Should we enable 5 level paging for PV guest ? (You are discussing)
>> 2.  Should we make the 5 level paging and 4 level paging supported with one 
>> binary?
>> 3.  When should the 5 level EPT be enabled?
> 
> Isn't it as usual i.e. let's enable once the spec is published? Or could you
> elaborate any concern of asking this question?
> 
>> 4. Should we extend shadow page to support 5 level paging guest?
>> 
> 
> I don't think so. EPT has been there for so many years. We can assume
> EPT available on new platform which supports 5-level.

Because of the quadratic nature of HAP (i.e., number of lookups becomes N x M 
rather than just N), there are still workloads where shadow pagetables performs 
better than HAP.  On my list of things to do sometime when I have time:
* Implement native superpages for shadows (ie., the shadows themselves will 
have superpages)
* Implement a way of automatically switching between shadow and HAP based on 
the workload

As Jan says, the shadow case will be necessary if we do 5-level PV guests; and 
I think it is still an important use case even for HVM guests (although it 
certainly comes second to other features mentioned here).

 -George
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